Validation of the ELISA for measuring SLPI with proteinases. Recombinant human SLPI was mixed with varying concentrations of NE and assayed by ELISA to assess the recovery of SLPI in complex with the proteinase and in the presence of excess proteinase. The molar ratio of proteinase to SLPI is given along the x axis, and the percentage of SLPI recovered is shown on the y axis. The figure shows the results obtained with NE, CG, PPE and trypsin. The ELISA was affected by a large molar excess of proteinase, but not at the molar ratios relevant to this paper (< 10:1).